In my annotated bibliography, I have summarized each source regarding their theory, experiments and research. The evolutionary theory of aging proposes that senescence that occurs in human beings is because the force of natural selection weakens with age. Everyone ages whether it is fast or slow. As people age, they are more prone to diseases, there are more damages to the DNA repair mechanisms, there are more mutations in our genome, and telomerase enzymes no longer function which changes the structure of the chromosome and lose certain functions and expressions of the genes. Furthermore, as people age, reproductive success significantly declines. For these reasons, aging can be viewed as an unfavorable characteristic to organisms. Nevertheless, aging can be a favorable characteristic to some degree at different point in the lifecycle. As it mentions in all three papers, organisms may trade late survival for enhanced reproduction in earlier life. Moreover, a gene can have a pleiotropic effect where it can beneficial at one point in life and be harmful in another point in life. That gene can be helpful by having successful reproductions but also be harmful by affecting the trait to age. In order to be fit, people need to reproduce so they trade to live longer life in order to increase the chance of reproduction. They have a better chance to have healthy babies so that their progeny can pass on their genes onto the next generation and so on. Moreover, fewer people are alive when they are old whether it is due to an accident or disease or eaten by a predator. Plus, there is a loose genetic control over the later stages so their chance of reproductive success significantly decreases. Therefore, people reproduce when they are younger because they have better genetic control and regulations of their body. All in all, through this trade-off, fitness is maximized. In addition, when parents have less offspring, they tend to live longer. The three articles that I have chosen explain why we age and the positive and negative aspects of aging and reproduction.
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Williams, George. C. 1957. Pleiotropy, Natural Selection, and the Evolution of Senescence. Evolution 11: 398-411.
Williams proposes that senescence is one of the effects of natural selection for organisms to increase their fitness. There is a pleiotropic effect on a gene that results in advantageous outcome earlier in life and have disadvantageous outcome later in life. Accordingly, the will increase the chance of successful reproductions during the youthful years while triggering the process of aging. Just to enhance reproductive success, the genes act to becomes harmful later in life. Basically Williams argues that pleiotropy gene affects differently at different stages in an organism's lifecycle. Williams states his four assumptions to expand his theory.
"(1) A soma that is essential to reproductive success but no part of which is passed on in either sexual or asexual reproduction. (2) Natural selection of alternative alleles in a population. (3) Pleiotropic genes of a special sort. It is necessary to postulate genes that have opposite effects on fitness at different ages, or, more accurately, in different somatic environments. (4) Decreasing probably of reproduction with increasing adult age" (1957: 400).
As it is stated in assumption three, the genes control more than one trait that is beneficial to the individual's reproductive success, but also are harmful to the individual's lifespan. The gene has a alternative effects on fitness at different stages in an organism's life because the effects of a gene do not have to be always beneficial or harmful. It can be both just during different periods of one's life. This pleiotropic effect is shown in organisms like human beings because the increased probability of fitness of an individual outweighed the longevity of one's life. Furthermore, this pleiotropic effect is expressed because for any organism, as it ages, it decreases the probability of reproducing. In the paper, Figure 1 shows a reproductive probability distribution curve to explain this (Williams 1957: 400). Williams uses this explanation to show a possible reason for the outcome of the pleiotropy gene because the longer one lives, the chance of survival rate decreases. To better explain this, Williams uses an equation proposed by Wright (1956): "W= (1+ m1p1) (1+ m2p2)â€¦. (1+mnpn) where m is the magnitude of an effect and p is the relevant proportion of the reproductive probability" (Williams 1957: 401). (Please note that I have not read the paper by Wright and I'm only using the equation that Williams thought was useful to explain his fourth assumption). He states if the value of W is calculated at different ages, it will show that natural selection will always favor youth over old. As an organism gets older, it will have lower p-values which will decrease the value of W.
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Next, Williams provides explanations to various expectations that can be derived from his theory. For instance, one expectation states that reproducing is the only factor for aging. Nevertheless, Williams shows that reproducing is not the only condition for rate of aging. Physical attribute or an environmental niche determines the rate of aging by decreasing the death rates of an organism. Williams uses the studies done by Lack (1954) and Blair (1948, 1951) to illustrate that birds have lower adult mortality rates and greater potential to have a longer life than mammals of similar size. A study by Comfort (1956: 160) also shows that the differences in senescence rate are due to physical traits like wings to fly. (Please note that I have not read these papers, but using it as a reference because Williams uses these studies to explain the proposed expectation).They tested this result by showing that birds like ostrich and the emu have the lowest longevity even though they have similar reproductive processes like other birds. This is because they cannot fly whiles birds with wings have longer lifespan which showed that physical attributes are associated with low adult death rates. In addition, the shells of turtles confirm that they live longer than other reptiles like alligator because turtles are protected by the shell. These factors act to lower mortality rates for higher age groups and strengthen the selection against shorter life and reproducing is not the only reason for shorter life. For another expectation, Williams denies the idea that there is little or no post-reproductive period. Williams says that there are post-reproductive period for animals like rats and human beings. For example, females undergo menopause when they are about forty-five years or older. Menopause is not part of an aging syndrome but is a process to terminate the dangerous process of pregnancy. This shows that in their lifecycle they do have post-reproductive period. Williams reason that this is so that they can use the extra energy to better take care of their offspring and the offspring that come after. Humans have post-reproductive systems just in a different degree for women and men. Another expectation from the theory is that when organisms were artificially selected for longer lifespan, the organisms should live a longer life if it did not have an energetic and harmful life. He predicts that these artificial genes should have positive effect on longer life when organisms live a less vigor life.
The paper gives a possible theory to why humans age. Williams argues that there is an indirect selective force that acts to increase their rate of senescence by favoring strong, fertile, youthful individuals with the intention of increasing the success rates of reproduction. Humans have a pleiotropy gene that gets expressed in different periods of one's life. William's theory of evolution of senescence is very significant to evolutionary biology. He was able to explain the idea of aging through evolutionary perspective. His idea of pleiotropy successively explains why we might age even if human aging has a negative impact. Any organism's main goal in life is to reproduce so that they can pass on its genes to its future generations. As a result, to maximize this success, it was bound for mammals like humans to trade higher productive success for longevity. Since the total reproductive probability depends on the times of the effects, the best time period of reproductive success is favored. Organisms will evolve to increase the chance of reproductive success even if it is detrimental later in their life. However, Williams states that although natural selection will always try to decrease the rate of senescence, senescence is a natural consequence of natural selection for fitness. Through careful observations, he developed a possible theory of evolution of aging.
Blair, W. Frank. 1848. Population density, life span, and mortality rates of small mammals in the blue-grass meadow and blue-grass field associations of southern Michigan. American Midland Naturalist 40: 395-419.
Blair, W. Frank. 1951. Population structure, social behavior, and environmental relations in a natural population of the beach mouse (Peromyscus polionotus leucocephalus). Contr. Lab. Vertebr. Biol., Univ. of Mich. 48: 1-47.
Comfort, Alex. 1954. Biological aspects of senescence. Biological Reviews 29: 284-329.
Lack, David. 1954. The Natural Regulation of Animal Numbers. Clarendon Press, Oxford viii + 343pp.
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Wright, Sewall. 1956. Modes of selection. American Naturalist 90: 5-24.
Kirkwood T.B.L. 1997. The origins of human aging. Philosophical transaction of the royal society Biological Sciences 352: 1765-1772.
In the paper, Kirkwood attempts to explain the origin of aging. First, he states that aging is a natural characteristic of mammals and other class of organisms. He claims senescence occurs in organisms as a trade-off to invest more energy earlier in their lifetime to enhance reproduction. Nevertheless, he states that human still try to live a very long time so have other features that allows them to do so. He describes aging as an indirect action of natural selection where specific genes can be accumulated in the genome due to biological and social evolution. Moreover, Kirkwood explains that aging is a slow, deteriorating process of cellular mechanism resulting in a loss of adaptive response to environmental stress. Because aging is an unfavorable outcome, Kirkwood develops an explanation of why aging occurs by using the concept of pleiotropy of a gene being positive at one point in life and being negative at another point in life. He thinks that it is an evolutionary trade-off for organisms to age in order to have a better reproductive success. Nevertheless, humans also still try to maximize their life by having post reproductive period in their life cycle.
Kirkwood first explains the mechanism of aging. He refers to Sohal (1993) and Martin (1996) to say that one of the causes of aging is caused by constant oxidative damages which is caused by reactive oxygen. The free radicals are very reactive and it can easily damage the cells. The accumulation of damages in the cell can eventually lead to aging. Furthermore, mutations in the DNA can also lead to aging. It is considered to be one of the natural causes of aging because mutations alters the DNA sequences and interrupts normal cell functions (Hanawalt 1987; Vijg 1990). (Please note, that I have not read the paper, but only describing it through their experiments through Kirkwood's understanding of their experiment).
Kirkwood uses another study to say that natural selection will normally operate so that senescence has little or no impact on populations in the wild (Promislow & Harvey 1990). (Please note, that I have not read the paper, but only describing it through their experiments through Kirkwood's understanding of their experiment). Organisms will try its best to always live longer. This can be understood by studying women and menopause. Kirkwood explains that most women start menopause at the age of forty-five through fifty. Menopause shuts down the reproductive functions for women in order to compensate for all the energy put in at younger age for reproductive success and to now use that energy for survival of oneself and its progeny. During one's menstruation cycle, women use a lot of energy and so they tend to get tired and tend to feel fatigue so now women undergoing menopause will have extra energy to live longer. Post-reproductive life increases the likelihood of longer and healthier life. Next, Kirkwood proposes a theory of evolution of human longevity by using other sources to back up his idea. He first claims that the main reason that is responsible for increase in longevity is the increase in brain size and smaller environmental risks. Plus, the advanced DNA repair system along with social/cultural evolution increased human's life by allowing less mutations and helping and working out problems in a group. In conclusion, Kirkwood uses these studies to show that aging has band consequences like damaging the cell, but it is a worthwhile investment to increase the probability of successful reproduction. Nevertheless, to compensation for aging, humans evolved to live a longer life by having a bigger brain and having post-reproductive periods.
This article gave theoretical explanations of why humans age. Kirkwood shared a similar idea that it is a trade-off of longevity and reproductive success. Nevertheless, he adds another concept that humans still try its best to maximize its lifespan after reproduction. He explains in his idea of evolution of menopause that the process of menopause is interplay of biological and social evolution to save energy that was needed during the menstruation cycle. It gives extra energy to older women to finally take care of themselves and also be a better mother figure to their children and grandchildren. This paper is significant in that it incorporated different studies to explain biological origins of human aging and evolution of human longevity. Human population will always continue to evolve to maximize the fitness and survival skills. It illustrated how evolution can affect two contrasting processes in the body and how the body tries to balance the negative and the positive features to regulate maintenance functions of human beings.
Westendorp, Rudi G. J. and Kirkwood Thomas B. L. 1998. Human longevity at the cost of reproductive success. Nature 396: 743-746.
Through their statistical reasoning and analyzing data, Westendorp and Kirkwood concluded that for the majority of the population, the longest lived individuals have reduced fertility. To insure that they have fair sample population, they identified that socioeconomic conditions affect both longevity and reproduction as a result they chose to study the British aristocratic families because they pretty much have about the same lifestyles and conditions. Table 2 shows the age at death and number of progeny in aristocratic women. For example, data collected in 1851-1875 showed that out of 385 women, the average age of death was 68 years old and they averaged number of 1.54 progeny. However, in data collected in year 1701-1725, out of 124 women, the average age of death was 43.3 years old and had the greatest average of 2.50 progeny. There are other data in different time periods, but the general trend is that the more progeny women had, the earlier they died. The difference between the two groups shows significant result. The average number of progeny only differed by one progeny, but the difference between the average ages of death was about 25 years.
When females who died at age twenty or younger are eliminated in the data, nearly a third of all women and half of those who died above 80 years of age were childless. The calculated value of likelihood ratio is 24.2 with d.f. =3 and P <0.001. These values show that among women sixty years and over, the number of progeny was negatively correlated which means as the number of progeny increased, female longevity decreased. Alternatively, the age at first childbirth was positively correlated with longevity meaning both values increased together (F=4.08; d.f. = 3,516; P=0.007).
When only postmenopausal women were included in the analysis, the decrease in the number of progeny with age at death was statistically significant for both the years pre-1700 and years between1700-1875. The results showed that the likelihood ratio of 21.4 and 12.9 respectively with three degrees of freedom. They also studied men and men showed a similar trend between reproduction and longevity as women. With this, Westendorp and Kirkwood stated that the force of natural selection progressively weakens with increasing adult age. In Figure 2, it shows that there is a stronger relationship between fertility and longevity for women born before 1700. They think that there may be a trade-off between reproductive success and longevity because resources invested in longevity assurance may be at the expense of reproduction. These mechanisms are not mutually exclusive, and generalized trade-off s between early and late life fitness are also implied by the pleiotropic gene hypothesis (Williams 1957).
This statistical study by Westendorp and Kirkwood is also significant because it further confirms that those with about the same environmental conditions and lifestyle live longer if the number of their progeny decrease. This is explicable because it takes a lot of time, effort, and energy to raise and have a child. Therefore, the more one reproduce, the less it is likely for them to live longer than those who do not reproduce as much. The evolutionary trade-off of longevity and higher reproductive success is validated through this experiment. A lot of energy and work is put in to reproduce so when a person does not reproduce, s/he saves that energy to better maintain their somatic and cellular functions.